Preferred embodiment to led light string

ABSTRACT

An LED light string employs a plurality of LEDs wired in block series-parallel, where the one or more series blocks, each driven at the same input voltage as the source voltage (110 VAC or 220 VAC), are coupled in parallel. The LED light string interfaces to the source voltage using a common household plug; it may also include a corresponding common, household socket, coupled in electrical parallel, to enable multiple light strings to be connected to each other from end to end. In order to directly drive a network of diodes without current-limiting circuitry, the voltage of each series block of diodes must be matched to the input source voltage. This voltage matching requirement for direct AC drive places fundamental restrictions on the number of diodes on each diode series block, depending on the types of diodes used. For the voltage to be “matched,” in each series block, the peak input voltage must be less than or equal to the sum of the maximum diode voltages for each series block.

CROSS REFERENCES TO RELATED APPLICATIONS

[0001] This application is a continuation of copending application Ser.No. 09/819,736 filed Mar. 29, 2001, which is a continuation-in-part ofcopending application Ser. No. 09/378,631 filed Aug. 20, 1999, titledPreferred Embodiment to Led Light String, which is acontinuation-in-part of copending application Ser. No. 09/339,616 filedJun. 24, 1999, titled Preferred Embodiment to Led Light String, which isa continuation-in-part of copending application Ser. No. 09/141,914filed Aug. 28, 1998, titled Led Light String Employing Series-parallelBlock Coupling. The disclosures of the aforementioned applications areincorporated herein by reference. This application claims benefit ofU.S. Provisional Application No. 60/119,804, filed Feb. 12, 1999.

BACKGROUND OF THE INVENTION

[0002] 1. Field of Invention

[0003] The present invention relates to light strings and, moreparticularly, to decorative light strings employing LEDs.

[0004] 2. Description of Related Art

[0005] Light emitting diodes (LEDs) are increasingly employed as a basiclighting source in a variety of forms, including decorative lighting,for reasons among the following. First, as a device, LEDs have a verylong lifespan, compared with common incandescent and fluorescentsources, with typical LED lifespan at least 100,000 hours. Second, LEDshave several favorable physical properties, including ruggedness, cooloperation, and ability to operate under wide temperature variations.Third, LEDs are currently available in all primary and several secondarycolors, as well as in a “white” form employing a blue source andphosphors. Fourth, with newer doping techniques, LEDs are becomingincreasingly efficient, and colored LED sources currently available mayconsume an order of magnitude less power than incandescent bulbs ofequivalent light output. Moreover, with expanding applications andresulting larger volume demand, as well as with new manufacturingtechniques, LEDs are increasingly cost effective.

[0006] LED-based light strings, used primarily for decorative purposessuch as for Christmas lighting, is one application for LEDs. Forexample, U.S. Pat. No. 5,495,147 entitled LED LIGHT STRING SYSTEM toLanzisera (hereinafter “Lanzisera”) and U.S. Pat. No. 4,984,999 entitledSTRING OF LIGHTS SPECIFICATION to Leake (hereinafter “Leake”) describedifferent forms of LED-based light strings. In both Lanzisera and Leake,exemplary light strings are described employing purely parallel wiringof discrete LED lamps using a step-down transformer and rectifier powerconversion scheme. These and all other LED light string descriptionsfound in the prior art convert input electrical power, usually assumedto be the common U.S. household power of 110 VAC to a low voltage,nearly DC input.

SUMMARY OF THE INVENTION

[0007] The present invention relates to a light string, including a pairof wires connecting to a standard household AC electrical plug; aplurality of LEDs powered by the pair of wires, wherein the LEDs areelectrically coupled in series to form at least one series block;multiple series blocks, if employed, that are electrically coupled inparallel; a standard household AC socket at the opposite end forconnection of multiple light strings in an end-to-end, electricallyparallel fashion.

[0008] It is an object of this invention to provide a method andpreferred embodiment that matches the AC voltage rating of the LEDscoupled in series to the AC power input without the need for additionalpower conversion.

[0009] The present invention relaxes the input electrical powerconversion and specifies a preferred embodiment in which the LED lightstring is electrically powered directly from either a common household110 VAC or 220 VAC source, without a different voltage involved viapower conversion. The LEDs may be driven using household AC, rather thanDC, because the nominal LED forward bias voltage, if used in reversebias fashion, is generally much lower than the reverse voltage where theLED p-n junction breaks down. When LEDs are driven by AC, pulsed lightis effected at the AC rate (e.g., 60 or 50 Hz), which is sufficientlyhigh in frequency for the human eye to integrate and see as a continuouslight stream.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Other aspects, features and advantages of the present inventionwill become more fully apparent from the following detailed description,the appended claims, and the accompanying drawings in which:

[0011]FIGS. 1A and 1B show two example block diagrams of the lightstring in its embodiment preferred primarily, with one diagram for a 110VAC common household input electrical source (e.g., 60 Hz) and onediagram for a 220 VAC common household (e.g., 50 Hz) input electricalsource.

[0012]FIG. 2A shows a schematic diagrams of an embodiment of thisinvention in which the diodes of the 50 LEDs (series) blocks 102 of FIG.1 are connected in the same direction.

[0013]FIG. 2B Shows a schematic diagrams of an embodiment of thisinvention in which the diodes of the 50 LEDs (series) blocks 102 of FIG.1 are connected in the reverse direction.

[0014]FIGS. 3A and 3B show two example block diagrams of the lightstring in its embodiment preferred alternatively, with one diagram for a110 VAC common household input electrical source (e.g., 60 Hz) and onediagram for a 220 VAC common household (e.g., 50 Hz) input electricalsource.

[0015]FIG. 4 shows an example schematic diagram of the AC-to-DC powersupply corresponding to the two block diagrams in FIG. 3 for either the110 VAC or the 220 VAC input electrical source.

[0016]FIGS. 5A and 5B show example pictorial diagrams of themanufactured light string in either its “straight” or “curtain” form(either form may be manufactured for 110 VAC or 220 VAC input).

[0017]FIG. 6 shows an example pictorial diagram of special tooling ofthe housing for an LED housing in the light string, for assurance ofproper LED electrical polarity throughout the light string circuit.

[0018]FIG. 7 shows an example pictorial diagram of special tooling andmanufacturing of the LED and its housing in the light string, forassurance of proper LED polarity using the example in FIG. 6.

[0019]FIG. 8 shows an example pictorial diagram of a fiber optic“icicle” attached to an LED and its housing in the light string, wherethe “icicle” diffuses the LED light in a predetermined manner.

[0020]FIG. 9 is a graph of current versus voltage for diodes andresistors.

[0021]FIGS. 10A and 10B are a schematic and block diagrams of directdrive embodiments.

[0022]FIG. 11 is a plot showing the alternating current time response ofa diode.

[0023]FIG. 12 is a graph showing measured diode average current responsefor alternating current and direct current.

[0024]FIG. 13 is a graph showing measured AllnGaP LED average andmaximum AC current responses.

[0025]FIG. 14 is a graph showing measured light output power as afunction of LED current.

[0026]FIG. 15 is a graph showing measured GaAlAs LED average and maximumAC current responses.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0027] The term “alternating current voltage”, sometimes abbreviated as“VAC”, as used herein occasionally refers to a numerical amount ofvolts, for example, “220 VAC”. It is to be understood that the statednumber of alternating current volts is the nominal voltage which cyclescontinuously in forward and reverse bias and that the actualinstantaneous voltage at a given point in time can differ from thenominal voltage number.

[0028] In accordance with the present invention, an LED light stringemploys a plurality of LEDs wired in series-parallel form, containing atleast one series block of multiple LEDs. The series block size isdetermined by the ratio of the standard input voltage (e.g., either 110VAC or 220 VAC) to the drive voltage(s) of the LEDs to be employed(e.g., 2 VAC). Further, multiple series blocks, if employed, are each ofthe same LED configuration (same number and kinds of LEDs), and arewired together along the string in parallel. LEDs of the light stringmay comprise either a single color LED or an LED including multiplesub-dies each of a different color. The LED lenses may be of any shape,and may be either clear, clear-colored, or diffuse-colored. Moreover,each LED may have internal circuitry to provide for intermittent on-offblinking and/or intermittent LED sub-die color changes. Individual LEDsof the light string may be arranged continuously (using the same color),or periodically (using multiple, alternating CIP colors), orpseudo-randomly (any order of multiple colors). The LED light string mayprovide an electrical interface to couple multiple lights stringstogether in parallel, and physically from end to end. Fiber opticbundles or strands may also be coupled to individual LEDs to diffuse LEDlight output in a predetermined manner.

[0029] An LED light string of the present invention may have thefollowing advantages. The LED light string may last far longer andrequire less power consumption than light strings of incandescent lamps,and they may be safer to operate since less heat is generated. The LEDlight string may have reduced cost of manufacture by employingseries-parallel blocks to allow operation directly from a standardhousehold 110 VAC or 220 VAC source, either without any additionalcircuitry (AC drive), or with only minimal circuitry (DC drive). Inaddition, the LED light string may allow multiple strings to beconveniently connected together, using standard 110 VAC or 220 VAC plugsand sockets, desirably from end-to-end.

[0030] Direct AC drive of LED light string avoids any power conversioncircuitry and additional wires; both of these items add cost to thelight string. The additional wires impose additional mechanicalconstraint and they may also detract aesthetically from the decorativestring. However, direct AC drive results in pulsed lighting. Althoughthis pulsed lighting cannot be seen at typical AC drive frequencies(e.g. 50 or 60 Hz), the pulsing apparently may not be the most efficientuse of each LED device because less overall light is produced than ifthe LEDs were continuously driven using DC. However, this effect may becompensated for by using higher LED current during each pulse, dependingon the pulse duty factor. During “off” times, the LED has time to cool.It is shown that this method can actually result in a higher efficiencythan DC drive, depending on the choice of AC current.

[0031]FIG. 1 shows the embodiment of an LED light string in accordancewith the present invention, and as preferred primarily through AC drive.In FIG. 1, the two block diagrams correspond to a exemplary stringemploying 100 LEDs, for either 110 VAC (top diagram) or 220 VAC (bottomdiagram) standard household current input (e.g., 50 or 60 Hz). In thetop block diagram of FIG. 1, the input electrical interface consistsmerely of a standard 110 VAC household plug 101 attached to a pair ofdrive wires.

[0032] With the average LED drive voltage assumed to be approximately2.2 VAC in FIG. 1, the basic series block size for the top blockdiagram, corresponding to 110 VAC input, is approximately 50 LEDs. Thus,for the 110 VAC version, two series blocks of 50 LEDs 102 are coupled inparallel to the drive wires along the light string. The two drive wiresfor the 110 VAC light string terminate in a standard 110 VAC householdsocket 103 to enable multiple strings to be connected in parallelelectrically from end-to-end.

[0033] In the bottom block diagram of FIG. 1, the input electricalinterface likewise consists of a standard 220 VAC household plug 104attached to a pair of drive wires. With again the average LED drivevoltage assumed to be approximately 2.2 VAC in FIG. 1, the basic seriesblock size for the bottom diagram, corresponding to 220 VAC input, is100 LEDs. Thus, for the 220 VAC version, only one series block of 100LEDs 105 is coupled to the drive wires along the light string. The twodrive wires for the 220 VAC light string terminate in a standard 220 VAChousehold socket 106 to enable multiple strings to be connected inparallel from end-to-end. Note that for either the 110 VAC or the 220VAC light string, the standard plug and socket employed in the stringvaries in accordance to the country in which the light string isintended to be used.

[0034] Whenever AC drive is used and two or more series are incorporatedin the light string, the series blocks may each be driven by either thepositive or negative half of the AC voltage cycle. The only requirementis that, in each series block, the LEDs are wired with the samepolarity; however the series block itself, since driven in parallel withthe other series blocks, may be wired in either direction, using eitherthe positive or the negative half of the symmetric AC electrical powercycle.

[0035]FIGS. 2A and 2B show two schematic diagram implementations of thetop diagram of FIG. 1, where the simplest example of AC drive is shownthat uses two series blocks of 50 LEDs, connected in parallel andpowered by 110 VAC. In the top schematic diagram of FIG. 2A both ofthese LED series blocks are wired in parallel with the polarity of bothblocks in the same direction (or, equivalently, if both blocks werereversed). With this block alignment, both series blocks flash onsimultaneously, using electrical power from the positive (or negative,if both blocks were reversed) portion of the symmetric AC power cycle. Apossible advantage of this configuration is that, since the LEDs allflash on together at the cycle rate (60 Hz for this example), when thelight string flashes on periodically, it is as bright as possible.

[0036] The disadvantage of this configuration is that, since both blocksflash on simultaneously, they both draw power at the same time, and themaximum current draw during this time is as large as possible. However,when each flash occurs, at the cycle rate, the amount of light flashedis maximal. The flash rate, a 50-60 Hz, cannot be seen directly by humaneye and is instead integrated into a continuous light stream.

[0037] The bottom schematic diagram FIG. 2B, shows the alternativeimplementation for the top diagram of FIG. 1, where again, two seriesblocks of 50 LEDS are connected in parallel and powered by 110 VAC.

[0038] In this alignment, the two series blocks are reversed, relativeto each other, in polarity with respect to the input AC power. Thus, thetwo blocks flash alternatively, with one block flashing on during thenegative portion of each AC cycle. The symmetry, or “sine-wave” natureof AC allows this possibility. The advantage if is that, since eachblock flashes alternatively, drawing power during opposite phases of theAC power, the maximum current draw during each flash is only half ofthat previously (i.e., compared when both blocks flash simultaneously).However, when each flash occurs, at twice the cycle rate here, theamount of light flashed is reduced (i.e., half the light than if twoblocks were flashing at once as previously illustrated). The flash rate,at 100-120 Hz, cannot be seen directly by the human eye and is insteadintegrated into a continuous light stream.

[0039] The trade-off between reversing series blocks when two or moreexist in an AC driven circuit is influenced primarily by the desire tominimize peak current draw. A secondary influence has to do with theproperties of the human eye in integrating periodic light flashes. It iswell known that the human eye is extremely efficient in integratinglight pulses rapid enough to appear continuous. Therefore, the secondform of the light string is preferred from a power draw standpointbecause the effect on human perception is insignificant.

[0040] For AC drive with non-standard input (e.g., three-phase AC) theseries blocks may similarly be arranged in polarity to divide poweramong the individual cycles of the multiple phase AC. This may result inmultiple polarities employed for the LED series blocks, say threepolarities for each of the three positive or negative cycles.

[0041] As an alternative preference to AC drive, FIG. 3 shows two blockdiagrams that correspond to a exemplary string employing 100 LEDs and DCdrive, for either 110 VAC (top diagram) or 220 VAC (bottom diagram)standard household current input (e.g., 50 or 60 Hz). In the top blockdiagram of FIG. 3, the input electrical interface consists of a standard110 VAC household plug 301 attached to a pair of drive wires, followedby an AC-to-DC converter circuit 302. As in FIG. 1, with the average LEDdrive voltage assumed to be approximately 2.2 VAC in FIG. 3, the basicseries block size for the top block diagram, corresponding to 110 VACinput, is approximately 50 LEDs. Thus, for the 110 VAC version, twoseries blocks of 50 LEDs 303 are coupled in parallel to the output ofthe AC-to-DC converter 302 using additional feed wires along the lightstring. The two drive wires for the 110 VAC light string terminate in astandard 110 VAC household socket 304 to enable multiple strings to beconnected in parallel electrically from end-to-end.

[0042] In the bottom block diagram of FIG. 3, the input electricalinterface likewise consists of a standard 220 VAC household plug 305attached to a pair of drive wires, followed by an AC-to-DC convertercircuit 306. With again the average LED drive voltage assumed to beapproximately 2.2 VAC in FIG. 3, the basic series block size for thebottom diagram, corresponding to 220 VAC input, is 100 LEDs. Thus, forthe 220 VAC version, only one series block of 100 LEDs 307 is coupled tothe output of the AC-to-DC converter 307 using additional feed wiresalong the light string. The two drive wires for the 220 VAC light stringterminate in a standard 220 VAC household socket 308 to enable multiplestrings to be connected in parallel from end-to-end. Note that foreither the 110 VAC or the 220 VAC light string, the standard plug andsocket employed in the string varies in accordance to the country inwhich the light string is intended to be used.

[0043]FIG. 4 shows an example schematic electrical diagram for theAC-to-DC converter employed in both diagrams of FIG. 3. The AC input tothe circuit in FIG. 1 is indicated by the symbol for an AC source 401. Avaristor 402 or similar fusing device may optionally be used to ensurethat voltage is limited during large power surges. The actual AC to DCrectification is performed by use of a full-wave bridge rectifier 403.This bridge rectifier 403 results in a rippled DC current and thereforeserves as an example circuit only. A different rectification scheme maybe employed, depending on cost considerations. For example, one or morecapacitors or inductors may be added to reduce ripple at only minor costincrease. Because of the many possibilities, and because of theirinsignificance, these and similar additional circuit features have beenpurposely omitted from FIG. 4.

[0044] For either the 110 VAC or the 220 VAC version of the LED lightstring, and whether or not an AC to-DC power converter is used, thefinal manufacturing may be a variation of either the basic “straight”string form or the basic “curtain” string form, as shown in the top andbottom pictorial diagrams in FIGS. 5A and 5B. In the basic “straight”form of the light string, the standard (110 VAC or 220 VAC) plug 501 isattached to the drive wires which provide power to the LEDs 502 via theseries-parallel feeding described previously. The two drive and otherfeed wires 503 are twisted together along the length of the light stringfor compactness and the LEDs 502 in the “straight” form are aligned withthese twisted wires 503, with the LEDs 502 spaced uniformly along thestring length (note drawing is not to scale). The two drive wires in the“straight” form of the light string terminate in the standard(correspondingly, 110 VAC or 220 VAC) socket 504. Typically, the LEDsare spaced uniformly every four inches.

[0045] In the basic “curtain” form of the light string, as shownpictorially in the bottom diagram of FIGS. 5A and 5B, the standard (110VAC or 220 VAC) plug 501 again is attached to the drive wires whichprovide power to the LEDs 502 via the series-parallel feeding describedpreviously. The two drive and other feed wires 503 are again twistedtogether along the length of the light string for compactness. However,the feed wires to the LEDs are now twisted and arranged such that theLEDs are offset from the light string axis in small groups (groups of 3to 5 are shown as an example). The length of these groups of offset LEDsmay remain the same along the string or they may vary in either aperiodic or pseudo-random fashion.

[0046] Within each group of offset LEDs, the LEDs 502 may be spaceduniformly as shown or they may be spaced nonuniformly, in either aperiodic or pseudo-random fashion (note drawing is not to scale). Thetwo drive wires in the “curtain” form of the light string also terminatein a standard (correspondingly 110 VAC or 220 VAC) socket 504.Typically, the LED offset groups are spaced uniformly every six inchesalong the string axis and, within each group, the LEDs are spaceduniformly every four inches.

[0047] In any above version of the preferred embodiment to the LED lightstring, blinking may be obtained using a number of techniques requiringadditional circuitry, or by simply replacing one of the LEDs in eachseries block with a blinking LED. Blinking LEDs are already available onthe market at comparable prices with their continuous counterparts, andthus the light string may be sold with the necessary (e.g., one or two)additional blinkers included in the few extra LEDs.

[0048] In wiring any version of the preferred embodiment to the lightstring, as described previously, it is critical that each LED is poweredusing the correct LED polarity. This equates to all feeds coming fromthe same drive wire always entering either the positive or the negativelead of each LED. Since the drive wires are AC, it does not matterwhether positive or negative is chosen initially; it is only importantall the LEDs in each series block have the same polarity orientation(either all positive first or all negative first). In order tofacilitate ease of proper manufacturing, as well as ease of proper LEDbulb replacement by the user, each LED and its assembly into its housingmay be mechanically modified to insure proper polarity. An example ofmechanical modification is shown in FIG. 6A, where the LED (shown at farleft with a rectangular, arched-top lens) is modified to include a keyedoffset 601 on its holder 606, and accordingly, the LED lamp base 605incorporates a notch 602 to accommodate this keyed offset. This firstpair of modifications, useful for manufacturing only, results in the LEDbeing properly mounted within its base to form replaceable LED lampbulb. In order to properly fit this replaceable LED lamp bulb into itsholder on the light string, the lamp base is also modified to include akeyed offset 603 on its base 605, and the lamp assembly holder 607 iscorrespondingly notched 604 for proper alignment. This second pair ofmodifications is useful in both manufacturing and by the user, forproperly placing or replacing the LED lamp bulb into its holder on thelight string. The LED lamp base and holder collectively form the LEDhousing. Note that such a mechanical arrangement makes it physicallyimpossible to incorrectly insert the LED. FIG. 6B is a top view of thelamp base taken along viewing line 6B-6B of FIG. 6A.

[0049] In manufacturing the above modification to assure proper LEDpolarity, it may be advantageous to build the LED mold such that twopiece replaceable LED lamp bulb described in FIG. 6 can be made in onestep as a single piece. This is illustrated in FIG. 7, where the singlepiece replaceable LED lamp bulb 701 has a single keyed offset to fitinto its notched lamp holder 702. Although this requires more elaboratemodification of the LED base, the resulting assembly is now composed oftwo, rather than three, LED pieces and as such, may allow the lightsstring to be made more rapidly and at lower cost.

[0050] Typically, the LEDs in the light string will incorporate a lensfor wide-angle viewing. However, it is also possible to attach fiberoptic bundles or strands to the LEDs to spatially diffuse the LED lightin a predetermined way for a desirable visual effect. In such case, theLED lens is designed to create a narrow-angle light beam (e.g., 20degree beamwidth or less) along its axis, to enable the LED light toflow through the fiber optics with high coupling efficiency. An exampleof the use of fiber optics is shown in FIG. 8, where a very lossy fiberoptic rod is employed with intention for the fiber optic rod to glowlike an illuminated “icicle.” In FIG. 8, the LED 801 and its housing 802may be attached to the fiber optic rod 803 using a short piece of tubing804 that fits over both the LED lens and the end of the fiber optic rod(note that the drawing is not to scale). An example design uses acylindrical LED lens with a narrow-angle end beam, where the diameter ofthe LED lens and the diameter of the fiber optic rod are the same (e.g.,5 mm or {fraction (3/16)} inches). The fiber optic rod 803 is typicallybetween three to eight inches in length and may be either uniform inlength throughout the light string, or the fiber optic rod length mayvary in either a periodic or pseudo-random fashion.

[0051] Although the fiber optic rod 803 in FIG. 8 could be constructedusing a variety of plastic or glass materials, it may be preferred thatthe rod is made in either a rigid form using clear Acrylic plastic orclear crystal styrene plastic, or in a highly flexible form using highlyplasticized Polyvinyl Chloride (PVC). These plastics are preferred forsafety, durability, light transmittance, and cost reasons. It may bedesirable to add into the plastic rod material either air bubbles orother constituents, such as tiny metallic reflectors, to achieve thedesigned measure of lossiness for off-axis glowing (loss) versus on-axislight conductance. Moreover, it is likely to be desirable to add UVinhibiting chemicals for longer outdoor life, such as a combination ofhindered amine light stabilizer (HALS) chemicals. The tubing 804 thatconnects the fiber optic rod 803 to its LED lens 801 may also made froma variety of materials, and be specified in a variety of ways accordingto opacity, inner diameter, wall thickness, and flexibility. Fromsafety, durability, light transmittance, and cost reasons, it may bepreferred that the connection tubing 804 be a short piece (e.g., 10 mmin length) of standard clear flexible PVC tubing (containing UVinhibiting chemicals) whose diameter is such that the tubing fits snuglyover both the LED lens and the fiber optic rod (e.g., standard walltubing with {fraction (1/4)} inch outer diameter). An adhesive may beused to hold this assembly more securely.

[0052] The method of determining and calculating the preferred LEDnetwork that provides stable and functioning operation will now bedescribed.

[0053] Many current-limiting designs use a single impedance element inseries between the LED network and the power supply. Current-saturatedtransistors are a less common method of current limiting. A resistor isoften used for the impedance element due to low cost, high reliabilityand ease of manufacture from semiconductors. For pulsed-DC or AC power,however, a capacitor or inductor may instead be used for the impedanceelement. With AC power, even though the waveform shape may be changed bycapacitors or inductors, the overall effect of these reactive elementsis basically the same as a resistor, in adding constant impedance to thecircuit due to the single AC frequency involved (e.g., 60 Hz). In anycase, the fundamental effect of current-limiting circuitry is topartially linearize or limit the highly nonlinear current versus voltagecharacteristic response curve of the diode, as shown in FIG. 9 for asingle resistor element.

[0054]FIG. 10 shows the preferred embodiment of the invention, wherein anetwork of diodes, consisting of LEDs, is directly driven by the ACsource without any current-limiting circuitry. The top diagram is ageneral schematic diagram showing M series blocks of LEDs directlyconnected in parallel to the AC source where, for the m-th series block,there are N_(m) {1≦m≦M} LEDs directly connected to each other in series.Also shown is a reversal of polarity between some series blocks, placingthese blocks in opposite AC phase, in order to minimize peak current inthe overall AC circuit. The bottom diagram in FIG. 10 is a block diagramof the above schematic, where a combination plug/socket is drawnexplicitly to show how multiple devices can be directly connected eitheron the same end or in an end-to-end fashion, without additional powersupply wires in between. This end-to-end connection feature isparticularly convenient for decorative LED light strings.

[0055] The invention in FIG. 10 may have additional circuitry, notexplicitly drawn, to perform functions other than current-limiting. Forexample, logic circuits may be added to provide various types ofdecorative on-off blinking. A full-wave rectifier may also be used toobtain higher duty factor for the diodes which, without the rectifier,would turn on and off during each AC cycle at an invisibly high rate(e.g., 50 or 60 Hz). The LEDs themselves may be a mixture of any type,including any size, shape, material, color or lens. The only vitalfeature of the diode network is that all diodes are directly driven fromthe AC power source, without any form of current-limiting circuitry.

[0056] In order to directly drive a network of diodes withoutcurrent-limiting circuitry, the voltage of each series block of diodesmust be matched to the input source voltage. This voltage matchingrequirement for direct AC drive places fundamental restrictions on thenumber of diodes on each diode series block, depending on the types ofdiodes used. For the voltage to be “matched,” in each series block, thepeak input voltage, V_(peak), must be less than or equal to the sum ofthe maximum diode voltages for each series block. Mathematically, letV_(peak) be the peak voltage of the input source and let V_(max)(n,m) bethe maximum voltage for the n-th diode {1≦n≦N_(m)} of the m-th seriesblock {1≦m≦M}. Then, for each m, the peak voltage must be less than orequal to the m-th series block voltage sum, $\begin{matrix}{V_{peak} \leq {\sum\limits_{n}{V_{\max}\left( {n,m} \right)}}} & (1)\end{matrix}$

[0057] where {1≦n≦N_(m)} in the sum over n. For simpler cases where allN_(m) diodes in the m-th series block are of the same type, each withV_(max), then V_(peak)≦N_(m) V_(max).

[0058] The maximum voltage V_(max) of each diode is normally defined bythe voltage which produces diode maximum current, I_(max). However, whendiodes of different types are used in a series block, the series blockvalue of I_(max) is the minimum of all individual diode values forI_(max) in the series block. Thus, if the m-th series block has N_(m)diodes, with the n-th diode in the m-th series block having maximumcurrent I_(max)(n,m), then the value of I_(max) for the m-th seriesblock, I_(max)(m), is determined by the minimum of these N_(m)individual diode values,

I _(max)(m)=min[I _(max)(n,m); {1≦n≦N _(m)}]  (2)

[0059] The maximum voltage V_(max) of each diode in the m-th seriesblock is thus defined as the voltage which produces the m-th seriesblock maximum current I_(max)(m). For simpler cases where all diodes ina series block are of the same type, each with maximum current I_(max),then I_(max)(m)=I_(max).

[0060] For AC or any other regularly varying input voltage, there is anadditional requirement to direct drive voltage matching. Here, in asimilar way to peak voltage above, the average, or RMS, voltage of thesource, V_(rms), must also be less than or equal to the sum of theaverage diode voltages, V_(avg), for each series block. Mathematically,let V_(rms) be the RMS voltage of the input source and let V_(avg)(n,m)be the average forward voltage for the n-th diode {1≦n≦N_(m)} of them-th series block {1≦m≦M}. Then, for each m, the RMS voltage must beless than or equal to the m-th series block voltage sum, $\begin{matrix}{V_{r\quad m\quad s} \leq {\sum\limits_{n}{V_{avg}\left( {n,m} \right)}}} & (3)\end{matrix}$

[0061] where {1≦n≦N_(m)} in the sum over n. For simpler cases where allN_(m) diodes in the m-th series block are of the same type, each withV_(rms), then V_(rms)≦N_(m) V_(avg).

[0062] In a similar way to the peak voltage above, the average voltageof each diode, V_(avg) is normally defined by the voltage which producesdiode average current, I_(avg). However, when diodes of different typesare used in a series block, the series block value of I_(avg) is theminimum of all individual diode values for I_(avg) in the series block.Thus, if the m-th series block has N_(m) diodes, each with averagecurrent I_(avg)(n,m) then the value of I_(avg) for the M-th seriesblock, I_(avg)(m), is determined by the minimum of these N_(m) values,

I _(avg)(m)=min[I _(avg)(n,m); {1≦n≦N _(m)}].  (4)

[0063] The average voltage V_(avg) of each diode in the m-th seriesblock is thus defined as the voltage which produces the m-th seriesblock average current I_(avg)(m). For simpler cases where all diodes ina series block are of the same type, each with average current I_(avg),then I_(avg)(m)=I_(avg).

[0064] Note that the term “average”, rather than “RMS,” is used todistinguish RMS diode values from RMS input voltage values because diodevalues are always positive (nonnegative) for all positive or negativeinput voltages considered, so that diode RMS values are equal to theirsimple averages. Note also that in past LED designs, the specified DCvalue for I_(nom) is equated to the average diode value, I_(avg). LEDsare always specified in DC, and the specified DC value for I_(nom)results from a tradeoff between LED brightness and LED longevity. In thedirect AC drive analysis below, this tradeoff between brightness andlongevity results in values for I_(avg) that are generally differentthan I_(nom). The direct AC drive value for V_(avg) is thus alsogenerally different than the LED specified DC value V_(nom).

[0065] LEDs are specified in terms of DC values, V_(nom) and I_(nom).For AC power, since V_(avg) is an AC quantity and V_(nom) is a DCquantity, they are fundamentally different from each other. This basicdifference between AC and DC values arises from the nonlinearrelationship between diode voltage and diode current. Consider ACvoltage input to a diode as shown for one period in FIG. 11, where thepeak voltage shown, V_(pk), is less than or equal to the diode maximumvoltage, V_(max). For AC voltages below the diode voltage threshold,V_(th), the current is zero. As the voltage increases above V_(th) toits peak value, V_(pk), and then falls back down again, the diodecurrent rises sharply in a nonlinear fashion, in accordance to itscurrent versus voltage characteristic response curve, to a peak value,I_(pk), and then the diode current falls back down again to zero currentin a symmetric fashion. Since the voltage was chosen such thatV_(pk)≦V_(max), then the peak diode current satisfies I_(pk)≦I_(max).The average diode current, I_(avg), is obtained by integrating the areaunder the current spike over one full period.

[0066] The central problem of AC voltage matching in equations (1)through (4) for direct drive of diodes is to first determine peak ACdiode current, I_(peak) and average AC diode current, I_(avg), as afunction of V_(rms) or, equivalently, the peak AC voltageV_(peak)={square root}2 V_(rms). Since the nonlinear relationship fordiode current versus voltage is not known in closed form, these diode ACcurrent versus input AC voltage relationships cannot be obtained inclosed form. Moreover, the nonlinear diode AC current versus input ACvoltage relationships vary for different diode types and materials. Inall cases, since the diode current versus voltage characteristic curve,near the practical operating point V_(nom), is a convex-increasingfunction, i.e., its slope is positive and increases with voltage, theaverage diode current I_(avg) that results from a given RMS value of ACvoltage is always higher than the diode current that would be achievedfor a DC voltage input having the same value. Because of this, specifiedDC values for diode voltage cannot be directly substituted for AC diodevoltage values. Instead, the characteristic diode AC current versusinput AC voltage relationships must be found for the AC waveform ofinterest.

[0067] The characteristic diode AC current versus voltage relationshipsmay be found by measuring diode current values I_(avg) and I_(peak) as afunction of RMS voltage, V_(rms), using variable voltage AC source. Anumber of alike diodes are used in these measurements to obtain goodstatistics. If different diode types or materials are considered, theneach measurement procedure is repeated accordingly. FIG. 12 shows atypical measurement result for average current, I_(avg), where the diodeused has specified nominal values of V_(nom)=2 VDC and I_(nom)=20 mA.

[0068] The average AC current curve is always to left of the DC currentcurve in FIG. 12. Thus, FIG. 12 shows that if one used DC voltages forthe diode in an AC circuit, the resulting average AC diode current wouldbe much higher than the DC current expected. Recall that in the priorart, where a number of alike 2 VDC LEDs are connected in series with acurrent-limiting resistor, a maximum number N of LEDs is defined bysumming the individual LED voltages and equating to the RMS inputvoltage. For a 120 VAC source, this maximum number is N=60 LEDs. Theprior art then subtracts five or ten LEDs from this maximum to obtain adesign number, and computes the resistor value using the differencebetween the AC input RMS voltage and the sum of these DC LED voltages.This design is marginally stable, and then becomes unstable, as thenumber of LEDs subtracted becomes smaller. Instability is proven in FIG.12, by considering the limit case where a maximum number N=60 of LEDsare used and hence no LEDs are subtracted. In this limit case, one mightargue that a resistor must be used anyway, but according to this designformula, presented for five or ten LEDs subtracted, the resistor valuein this case would equal zero. As FIG. 12 shows, if the resistor valuewere zero, i.e., the resistor is omitted, instead of the DC design valueof I_(nom)=20 mA for LED current (the rightmost, DC, curve at 2.0 VDC),the LED average AC current will be off the scale, higher than themaximum diode current I_(max)=100 mA (the leftmost, AC, curve at 1.87VAC), and the device will fail immediately or almost immediately.

[0069] In order to properly perform matching in an direct AC drivedesign, the characteristic diode AC current versus input AC voltagerelationships must be measured and used to specify the AC values forequations (1) through (4). DC specifications and DC diode measurementscannot directly be used in the direct AC drive design, and they areuseful only as a guide for theoretical inference, discussed furtherbelow. Along with the diode average AC current, the diode peak ACcurrent must also be measured as a function of RMS (or equivalently,peak) input AC voltage. FIG. 13 shows a typical measurement result,where the diode used has specified DC nominal values of V_(nom)=2 VDCand I_(nom)=20 mA.

[0070] As stated previously, for an AC design, the LED average ACcurrent, I_(avg), is generally different from the specified LED nominalDC current, I_(nom). Likewise, the LED maximum AC current, I_(max), isalso generally different from the specified LED maximum DC current.Choice of these values represent a tradeoff between LED brightness andelectrical efficiency versus LED longevity. In general for pulsed-DC orAC input, the LED is off at least part of the time and is therefore hastime to cool during off-time while heating during on-time. In order toincrease light output and hence electrical efficiency, both the averageand the peak diode current values can be raised somewhat above specifiedDC values and maintain the same longevity, which is defined as the totalon-time until, say, 30% loss of light output is incurred—typically atabout 100,000 on-time hours. Moreover, these LED average and peakcurrent values can be raised further to increase light output andelectrical efficiency at some expense in LED longevity, depending on theon-time duty factor. Higher ambient temperatures are accounted for bylowering, or “derating” these values somewhat.

[0071] In a publication by Hewlett Packard, a number of curves arepresented of projected long term light output degradation, for variouspulsed-DC duty factors and various average and peak current values, atambient temperature T_(A)=55° C. The AlInGaP LEDs used in this datarepresents the material commonly used in an LED with specified DCnominal voltage V_(nom)=2 VDC. While results vary somewhat for other LEDmaterials, it can be inferred from this data that, for most LEDsspecified at I_(nom)=20 mA, the AC design choice for I_(avg) isapproximately in the interval,

30 mA≦I _(avg)≦50 mA  (5)

[0072] where the specific value chosen, I_(avg)=36 mA, is indicated inFIG. 13.

[0073] Similarly, from the Hewlett Packard data it can be inferred that,for most LEDs with maximum DC current specified at 100 mA, and the ACdesign choice for I_(max) is approximately,

I _(max)≦120 mA  (6)

[0074] where a specific value chosen of I_(max)=95 mA satisfying this,that corresponds to V_(avg)=1.6 VAC and I_(avg)=36 mA, is also indicatedin FIG. 13.

[0075] To clarify the direct AC drive design, consider again the simplercase where all N LEDs in a series block are of the same type, with eachLED specified as before at V_(nom)=2 VDC and I_(nom)=20 mA. Moreover,let the input AC power be the U.S. standard value and assume V_(rms)=120VAC for voltage matching. With the above values for I_(max) and I_(avg)the maximum and average LED voltages, V_(max) and V_(avg), aredetermined using AC current versus voltage measurements in FIG. 13 andsimplified versions of equations (2) and (4), respectively. The minimumnumber N of LEDs is determined from these voltages using the inputvoltage V_(peak)={square root}2 V_(rms) and equations (1) and (3), formaximum and average voltage respectively. Since the value for I_(max)=95mA was chosen as a lower value than possible by equation (6),corresponding to V_(avg)=1.6 VAC and I_(avg)=36 mA, the maximum voltagebecomes V_(max)={square root}2 V_(avg) and equation (1) is automaticallysatisfied by satisfying equation (3). Solving equation (3) results inthe minimum number of N LEDs as,

V _(rms) ≦N V _(avg)

120≦N(1.6)

N≧75  (7)

[0076] Although the value of N=75 is a convenient number to use formanufacturing and sale of a decorative LED light string, if a different,less convenient, minimum number N of LEDs were computed, the result canbe rounded up or down slightly for convenience, provided that thesubsequent changes in LED brightness or longevity are acceptable. Forexample, if the RMS voltage were assumed to be 110 VAC, then theresulting minimum number of LEDs in equation (7) would be N≧69, and thisvalue may be rounded to a final value of N=70 for convenience, with onlyslight impact on LED brightness.

[0077] Efficiency of the above direct AC drive design example can beestimated by first noting that the average power, P_(avg), consumed by asingle LED in the series block is the product of the average voltage andthe average current, P_(avg)=V_(avg) I_(avg). This is compared againstthe optimal DC baseline that uses the specified DC nominal LED powerconsumption, P_(nom), defined as the product of the nominal voltage andthe nominal current, P_(nom)=V_(nom) I_(nom). Using the values given inthe above direct AC drive example, there results, P_(avg)≈1.44 P_(nom),so that the direct AC drive design consumes 44% more power per LED thanthe DC baseline. However, to examine efficiency, first let L_(avg) bethe average light output power for the direct AC drive design and L_(DC)be the optimal light output power using the DC baseline. This lightoutput power L represents LED efficiency as a device, i.e., how muchlight the LED can be made to produce. Defining relative deviceefficiency as the quotient ε_(D)=L_(avg)/L_(DC) enables the amount oflight produced by each LED in direct AC drive design to be compared withthe optimal DC baseline. Using an approximation that the LED lightoutput power, L, is proportional to the LED current, I, this LED deviceefficiency, ε_(D), is approximately,

ε_(D) =L _(avg) /L _(DC) ≈I _(avg) /I _(nom)=36/20=1.8  (8)

[0078] so that the direct AC design example makes about 80% more use ofeach LED as a light producing device than the optimal DC baseline. Inother words, for each LED used, the direct AC drive design producesabout 80% more light than the maximum possible by a DC design based onnominal LED values. Although this factor of 80% light increase appearsto be large, its effect is diminished by human perception. According tothe well known law by Stevens, human perceptions follow a continuumgiven by the power relationship,

B∝L^(ρ)  (9)

[0079] where L is the stimulus power, B is the perceived brightnessintensity, and exponent ρ is a parameter that depends on the type ofstimulus. For light stimuli, L is the light power in Watts, B is theperceived photopic brightness in lumens, and the exponent isapproximately ρ≈⅓. With this exponent, the 80% increase in light outputpower offered by the direct AC design example translates into about 22%increase in perceived brightness. Although a smaller realized effect,the direct AC design example does offer an increase, rather than adecrease, in brightness relative to the optimal DC baseline.

[0080] LED electrical efficiency, E, is defined by dividing light outputpower by electrical power used, E=L/P. Defining relative electricalefficiency as the quotient ε_(E)=E_(avg)/E_(DC) enables the electricalefficiency in direct AC drive design to be compared with the optimal DCbaseline. Using again an approximation that the LED light output power,L, is proportional to the LED current, I, there follows,

ε_(E)≈(I _(avg) /P _(avg))/(I _(nom) /P _(nom))=V _(nom) /V_(avg)=2.0/1.6=1.25  (10)

[0081] so that the AC direct drive design is about 25% more electricallyefficient than the optimal DC baseline. In other words, for a fixedamount of input power, the direct AC design examples produces about 25%more light than the maximum possible by DC based on nominal LED values.

[0082] There are two basic reasons for the results in equations (8) and(10). First, the direct drive design does not have current-limitingcircuitry to consume power. If this were the only factor involved, thedirect AC design efficiency would be 100%, relative to the optimal DCbaseline, because the optimal DC baseline is computed withoutcurrent-limiting circuitry loss. The second basic reason stems from thenonlinear relationship between LED current and voltage. Because thisrelationship is a convex-increasing function, i.e., its slope ispositive and increases with voltage, average AC diode current I_(avg) isalways higher than DC current for the same voltage value. This higher ACaverage current in turn leads to higher average light output, with anapproximation showing a proportional relationship. This is a fundamentaladvantage to the pulsed waveforms over DC that others fail to recognizefor AC and instead try to avoid. The nonlinear current versus voltagerelationship is further taken advantage of in the direct AC drive designby increasing the average current to a more optimal value, using thefact that the LED has time to cool during the off-time interval in eachAC cycle.

[0083] An approximation that LED light output is proportional to LEDcurrent is very close for most operating values of LED current, but theapproximation usually overestimates light output at high current values.A typical curve for AlInGaP LEDs, the common material type for LEDs witha 2 VDC specification, is shown in FIG. 14. With this measured result,the relative direct AC drive efficiencies computed in equations (8) and(10) are lowered somewhat, but they are still well above unity. Anumerical integration using FIG. 14 indicates that equations (8) and(10) overestimate efficiency of the direct AC design in the examplepresented by about 15%, and closer estimates for the above relativeefficiencies are ε_(D)≈1.53 and ε_(E)≈1.06.

[0084] Diminishing light output power at high LED current places theoptimal value for RMS and peak LED current values, I_(avg) and I_(max),at a slightly lower value than the average and peak current constraintsin equations (5) and (6) allow. For example, FIG. 13 shows that thelargest value allowed by equations (5) and (6) for V_(avg) is 1.65 VAC,rather than the value of 1.60 VAC used above. This larger value ofV_(avg)=1.65 VAC, achieved by N=72 LEDs in a 120 VAC series block, isslightly less efficient, as well as slightly less reliable, than thevalue of V_(avg)=1.60 VAC and N=75 LEDs. However, the value of N=72 LEDsin the series block would cost less to produce per unit. Using 110 VACinstead of 120 VAC to obtain a lower number N=69 LEDs in the seriesblock yields yet slightly lower efficiency and reliability still. Fordecorative LED light strings, this final direct AC drive tradeoffbetween, say, 70 versus 75 LEDs in the series block exemplified is amatter of practical judgment to provide the highest quality product atthe lowest unit cost.

[0085] Although it has been shown above that LED specified DC valuescannot be directly used in for direct AC drive, these values do havesome theoretical utility for using a smaller measurement set to estimatethe AC design values. The theoretical basis of this estimation procedureresults from applying statistical inference on the LED specifications,using these specifications in a different way than they are obtained orintended.

[0086] LEDs are specified by two voltage parameters, a typical, or“nominal” voltage, V_(nom), and a largest, or “supremum” (usually called“maximum” by LED manufacturers) voltage, V_(sup). These specificationsare obtained as ensemble estimates, for a large ensemble of alike LEDs,of “typical” and “largest” DC voltages to expect, from variations due tomanufacturing, that produce the chosen nominal value of DC current,I_(nom). The nominal DC voltage, V_(nom), is intended as a “typical”value for the LED, obtained either by averaging measurements or bytaking the most likely, or modal, value in a measurement histogram. Themaximal DC voltage, V_(sup), is intended as a largest, or “supremum,”value for the LED, obtained by sorting the largest voltage valuemeasured that produces the chosen nominal value of DC current, I_(nom).

[0087] The theoretical problem of interest is to obtain values foraverage AC voltage, V_(avg), and maximum AC voltage, V_(max), thatproduce average AC current, I_(avg) and maximum AC current, I_(max),respectively. These voltage values V_(avg) and V_(max) do not considerLED ensemble variations due to manufacturing but instead rely on a largeenough number N of LEDs in each AC series block for manufacturingvariations to be averaged over. Otherwise, voltage equations (1) and (3)above must be altered slightly to account for expected LED manufacturingvariations. Such an alteration would rely on a statistical modelobtained by measuring variations of the characteristic AC current versusAC voltage curve, from LED to LED in a large ensemble of alike LEDs. Inany event, the voltages V_(avg) and V_(max) are fundamentally defined torepresent characteristic estimates of voltage for varying values of LEDcurrent, obtained by averaging over the ensemble, rather than ensembleestimates, using individual LEDs within the ensemble, of voltages thatproduce a fixed, say, nominal, value of LED current.

[0088] In order to make theoretical inferences from LED specifications,it must be assumed that the specified ensemble random variablesrepresenting “nominal” and “supremum” voltages can be interchanged withequivalent characteristic random variables representing correspondingvoltages that produce corresponding LED current over time. Thisassumption is similar to a commonly assumed form of ergodicity in randomprocess theory that interchanges ensemble random variables withcorresponding time-series random variables.

[0089] With this ergodicity assumption, the AC average and maximumvoltage values of interest, V_(avg) and V_(max), can be inferred fromthe specified diode values for DC nominal and maximum voltage, V_(nom)and V_(sup), respectively, using appropriate DC-to-AC scaling betweenthem. It is desired to obtain a single scale factor α for all LEDmaterials, colors, and LED manufacturers. In trying to find this singlevalue for scale factor α, difficulty arises in that the specifiedvoltages, V_(nom) and V_(sup), are fundamentally different for differentLED dopant materials. However, given a specific LED dopant material “M”,such as AlInGaP or GaAlAs, the variations in V_(nom) and V_(sup) acrossapplicable colors and manufacturers are small enough to be consideredfairly insignificant.

[0090] Recall that V_(max) is equated with peak input voltage V_(peak)in equation (1), and V_(avg) is equated with RMS input voltage V_(rms)in equation (3). For AC power, the quotient V_(peak)/V_(rms)={squareroot}2. It would thus be desirable if the quotient V_(sup)/V_(nom) werealso always a constant, preferably equal to {square root}2, so that asingle scale factor α_(M) could be used for each LED material, “M.”Unfortunately, this ratio also varies significantly for different LEDmaterials. As a result, two distinct scale factors α_(M) and β_(M) arerequired for each LED material composition, “M.” With thesematerial-dependent scale factors, α_(M) and β_(M), the AC voltages ofinterests are estimated from DC specified values using,

V _(avg)≈α_(M) V _(nom) , V _(max)≈β_(M) V _(sup).  (11)

[0091] where the scale factors α_(M) and β_(M) are determined bymeasurement. The advantage provided by this theoretical estimationprocedure is that the set of measurements determining characteristiccurves for peak and average AC current versus AC voltage need only beobtained for each LED dopant material, independent of LED color and LEDmanufacturer. Of course, the disadvantage to this procedure is that itis approximate when compared to making full measurement sets for allspecific types of LEDs considered, and hence some experimentation withthe exact number of LEDs is required.

[0092] For AlInGaP LEDs, V_(nom)=2.0 VDC and V_(sup)=2.4 VDC representthe centroids of specified values across applicable colors and fromvarious manufacturers. The characteristic curves presented in FIG. 7were obtained from AlInGaP LEDs. From FIG. 13, and the criteria foraverage and maximum AC current defined in equations (5) and (6),respectively, AC current values I_(avg)=36 mA and I_(max)=95 mA werechosen previously, with V_(max)={square root}2 V_(avg) and V_(avg)=1.6VAC. Equations (11), then, lead to α_(AlInGaP)=0.80 andβ_(AlInGaP)=0.94. These values may be used theoretically in equations(11) to estimate approximate AC average and maximum voltages, V_(avg)and V_(max) for other AlInGaP LEDs.

[0093]FIG. 15 shows measured characteristic curves for a different setof alike LEDs, where the dopant material is GaAlAs, rather than AlInGaP.For GaAlAs LEDs, V_(nom)=1.7 VDC and V_(sup)=2.2 VDC represent thecentroids of specified values across applicable colors and from variousmanufacturers. From FIG. 15, and the criteria for average and maximum ACcurrent defined in equations (5) and (6), respectively, AC currentvalues I_(avg)=38 mA and I_(max)=95 mA are chosen, with againV_(max)={square root}2 V_(avg), but now V_(avg)=1.45 VAC. Equations(11), then, lead to α_(GaAlAs)=0.85 and β_(GaAlAs)=0.93. These valuesmay be used theoretically in equations (11) to estimate approximate ACaverage and maximum voltages, V_(avg) and V_(max), for other GaAlAsLEDs. Note that, with 120 VAC assumed for the RMS input voltage, thisvalue V_(avg)=1.45 VAC leads to N=83 LEDs per series block. Similarly,with 110 VAC assumed for the RMS input voltage, N=76 LEDs per seriesblock. Rounding these values leads to either 75, 80, or 85 LEDs perseries block in a manufactured product, with N=75 being most desirablefor a decorative LED light string from a cost basis, if it issufficiently reliable.

[0094] The above direct AC drive design procedure has been verified bybuilding numerous decorative LED light string prototypes using a varietyof dopant materials, colors, and manufacturers. Many of these prototypeswere built as long as two years ago, and all prototypes have remainedoperating continuously without any sign of impending failure. Moreover,a number of these prototypes were subjected to harsh voltage surge andvoltage spike conditions. Voltage surge conditions were produced usinghigh power appliances in the same circuit, all of which failed toproduce anything other than at most some flickering. In about half ofthese experiments the voltage surges created caused circuit breakers totrip. The decorative LED light string prototypes, being waterproof, werealso immersed in water during testing.

[0095] Voltage spikes, simulating lightning discharges, were produced byinjecting 1000 V, 10 A pulses of up to 10 ms duration and one secondapart into a 100 A main circuit of a small home using a pulse generatorand 10 kW power amplifier. The amplifier was powered from the mainelectrical input of an adjacent home. During these tests, all decorativeLED light string prototypes merely flickered in periodic succession atone second intervals. In the meantime during these tests, the protectivecircuitry of adjoining electronic equipment shut off without any ensuingdamage. All these tests verified conclusively that the decorative LEDlight strings were designed to be highly reliable by the direct AC drivemethod, without the use of any current-limit circuitry.

[0096] It will be understood that various changes in the details,materials and arrangements of the parts which have been described andillustrated in order to explain the nature of this invention may be madeby those skilled in the art without departing from the principle andscope of the invention as expressed in the following claims.

1. A light string comprising: a predetermined number of light emittingdiodes (LEDs) and sockets forming individual electrical componentselectrically coupled in series to form at least one series block, eachelectrical component defining individual alternating current averagedrive voltages, the series block having a first electrical component anda last electrical component, and an alternating current electrical powersupply having an average supply voltage, wherein a summation of saidindividual alternating current average drive voltages is substantiallyequal to said average supply voltage.
 2. The light string of claim 1,where said first electrical component is directly coupled intermediate asource end and a terminal end of a first set of wires and the lastelectrical component directly coupled intermediate the source end andthe terminal end of a second set of wires, the light string being freefrom additional circuitry intermediate the first electrical componentand the source end of the first set of wires, between each of theelectrical components, and intermediate the last electrical componentand the source end of the second set of wires, and wherein a firstconnector is coupled to both the source end of the first set of wiresand the source end of the second set of wires which connectorfacilitates a direct connection between the first electrical componentand a first side of said alternating current electrical power supply,and the last electrical component and a second side of the alternatingcurrent electrical power supply.
 3. The light string of claim 1, whereinsaid predetermined number of electrical components substantially equalto the supply voltage divided by an average of said individualalternating current drive voltages.
 4. The light string of claim 1,wherein said individual alternating current drive voltages of said eachelectrical component is substantially the same.
 5. The light string ofclaim 1, wherein said individual alternating current drive voltages ofsaid each electrical component is substantially different.
 6. The lightstring of claim 1, wherein the supply voltage (V_(rms)) follows thefollowing relationship in order to determine the predetermined number ofLEDs:$V_{r\quad m\quad s} \leq {\sum\limits_{n}{V_{avg}\left( {n,m} \right)}}$

where m is a diode type and n is the predetermined number.
 7. The lightstring pf claim 6, where, when, all N_(m) diodes in the m-th seriesblock are of the same type, each with V_(rms), then V_(rms)≦N_(m)V_(avg).
 8. The light string of claim 1, in which each LED has a p-njunction defining a breakdown voltage above which voltage applied inreverse bias said p-n junction breaks down, and in which light stringalternating current is less than the break down voltage.
 9. The lightstring of claim 1, in which the first connector is polarized, and whichlight string further comprises a second polarized connector electricallyconnected to the terminal end of the second of the pair of wires, saidsecond polarized connector being adapted to couple with a firstpolarized connector of another light string, thereby providing forcoupling of multiple light strings in an end-to-end arrangement.
 10. Thelight string of claim 9, in which at least one of said LEDs comprises ahousing and a fiber-optic bundle removably mounted to the housingoperative to diffuse light output of the LED through the fiber-opticbundle.
 11. The light string of claim 1, in which the LEDs are offsetfrom the wires and arranged relative to a wire axis.
 12. The lightstring of claim 1, in which each LED is arranged parallel to the wiresto create a straight arrangement.
 13. The light string of claim 1, inwhich the LEDs in each series block are uniformly spaced apart.
 14. Thelight string of claim 1, further comprising a lamp holder having a keyedoffset, the lamp holder fixedly attached to each LED, and a lamp basehaving a notch adapted to receive the keyed offset of the lamp holder,thereby mechanically orienting and aligning each LED by its polarity.15. The light string of claim 14, wherein the lamp base furthercomprises a base keyed offset and a lamp assembly holder, the lampassembly holder having a notch adapted to receive the base keyed offset.16. The light string of claim 1, wherein the light string furthercomprises a plurality of series blocks.
 17. The light string of claim 1,further comprising a lossy fiber optic rod, having a diameter of acorresponding LED lens, and a fiber housing, wherein the fiber housingadaptively receives the rod and LED lens into opposing ends,cooperatively, thereby creating an optical icicle feature.
 18. A lightstring comprising: a predetermined number of light emitting diodes(LEDs) and sockets forming individual electrical components electricallycoupled in series to form at least one series block, each electricalcomponent defining individual alternating current average voltages andpeak voltages, the series block having a first electrical component anda last electrical component, and an alternating current electricalsupply having an average supply voltage and a peak supply voltage,wherein a first summation of said individual alternating current averagevoltages is substantially equal to said average supply voltage and asecond summation of said peak voltages is substantially equal to saidpeak supply voltage.
 19. The light string of claim 18, wherein saidindividual alternating current drive voltages of said each electricalcomponent is substantially the same.
 20. The light string of claim 18,wherein said individual alternating current drive voltages of said eachelectrical component is substantially different.
 21. The light string ofclaim 18, wherein the average supply voltage (V_(rms)) follows thefollowing relationship in order to determine the predetermined number ofLEDs: V _(rms)≦Σ_(n) V _(avg)(n,m) where m is a diode type and n is thepredetermined number.
 22. The light string of claim 21, where, when, allN_(m) diodes in the m-th series block are of the same type, each withV_(rms), then V_(rms)≦N_(m) V_(avg).
 23. The light string of claim 22,wherein the peak supply voltage (V_(peak)) follows the followingrelationship in order to determine the predetermined number of LEDs:$V_{peak} \leq {\sum\limits_{n}{V_{\max}\left( {n,m} \right)}}$

where m is a diode type and n is the predetermined number.
 24. The lightstring of claim 23, where, when, all N_(m) diodes in the m-th seriesblock are of the same type, each with V_(max), then V_(peak)≦N_(m)V_(max).